Easy, I found the statement copied below in their Terms of Service.
Processing Fees. We reserve the right to charge a processing fee to Political Committees and/or Page Creators to provide the Platform to you and to process transactions. You understand and acknowledge that the amount received by the Political Committee or Page Creator as a result of your contribution may be reduced by the amount of such processing fees.
Easy, of course I understand that it costs to provide a platform for all the candidates and/or their PACs, but I do wish they had stated the amount they take off the top.
They are vacuuming up a whole lot of data to sell, and I got hooked in by them last election cycle, not sure I want to do it again.
bkmk
When I donated, they wanted me to add on a $5 donation for processing. Jerks!